Method of making a color picture tube

ABSTRACT

IN MANUFACTURING A COLOR PICTURE TUBE, A PLURALITY OF COLOR POLSPHOR DOTS ARE PLACED ON A FACE PLATE OF THE TUBE. A SHADOW MASK USED WITH THE TUBE IS MADE WITH A PLURALITY OF APERTURES ARRANGED IN A PREDETERMINED PATTERN. EACH OF THESE APERTURES ARRANGED IN A PREDETERMINED PATTERN. PREDETERMINED SIZE OF EACH APERTUTE IS REDUCED TO A SMALLER SIZE WHEN THE MASK IS USED IN PLACING THE PHOSPHOR DOTS ON THE FACE PLATE. THIS SIZE OF EACH OF THE APERTURES IS RETURNED TO ITS ORIGINAL, PREDETERMINED SIZE WHEN THE MASK IS USED FOR OPERATING THE COLOR TUBE. THIS DISCLOSURE TEACHES AN IMPROVEMENT IN THE METHOD OF REDUCING AND, THEREAFTER, RETURNING THE SIZE OF THE APERTURES TO THE PREDETERMINED SIZE. THE IMPROVEMENT INCLUDES APPLYING A POWDER COATING COMPOSITION TO THE SHADOW MASK.

United States Patent US. Cl. 117-17 9 Claims ABSTRACT OF THE DISCLOSURE In manufacturing a color picture tube, a plurality of color phosphor dots are placed on a face plate of the tube. A shadow mask used with the tube is made with a plurality of apertures arranged in a predetermined pattern. Each of these apertures have a predetermined size. The predetermined size of each aperture is reduced to a smaller size when the mask is used in placing the phosphor dots on the face plate. The size of each of the apertures is returned to its original, predetermined size when the mask is used for operating the color tube. This disclosure teaches an improvement in the method of reduc ing and, thereafter, returning the size of the apertures to the predetermined size. The improvement includes applying a powder coating composition to the shadow mask.

BACKGROUND OF THE INVENTION Color television tubes are prepared by depositing a plurality of groups of color phosphor dots on a glass face plate of the tube. The size and location of the phosphor dots on the face plate are controlled by interposing a steel shadow mask having a plurality of apertures therein between a light source used in laying down the dots and the face plate. The same shadow mask is also used to direct electron beams to illuminate the dots during operation of the tube.

The best colors and brightest shades are obtained from a tube when different phosphor dots are separated from each other by dark areas. A shadow mask With apertures of 9 to 10 mils diameter is needed for depositing such well separated dots. During operation of the tube, however, the size of the shadow masks apertures must have a diameter in the range of 14 to 16 mils for transmission of a larger fraction of electron beam energy to illuminate fully the phosphor dots. The shadow masks apertures, therefore, are smaller in size during deposition of the phosphor dots than for operation of the tube. The prior art has solved this problem in many ways which are disclosed in such US. patents as 3,231,380 to Law, 3,574,013 to Frantzen, 3,604,081 to Moegenbier, and 3,616,732 to Rucinski.

The invention disclosed herein is an improvement in the method of reducing from a predetermined size the size of apertures of a shadow mask for deposition of color dots and for enlarging such apertures to the predetermined size for transmission of electron beams for illuminating the dots. The improved method employs a powder coating composition to reduce the size of the shadow masks apertures.

SUMMARY OF THE INVENTION This invention relates to a method of manufacturing a color picture tube and, more particularly, to an improvement in the method of manufacturing a color picture tube wherein a powder coating composition is employed in reducing the size of a shadow masks apertures so that the mask can be used in placing the color dots on the tube.

In the art of manufacturing a color picture tube having a plurality of groups of phosphor color dots placed on a face plate thereof, a metallic shadow mask is made with a plurality of apertures arranged in a predetermined pattern. Each of the apertures of the shadow mask has a predetermined size larger than the size of each of the dots to be laid down on the face plate of the tube. The predetermined size of these apertures is reduced to a smaller size and the phosphor color dots are placed on the face plate with the aid of the shadow mask having the smaller apertures therein. The size of the apertures of the shadow mask are returned to the predetermined size so that the same shadow mask can be used in operating the tube. The method of this invention sets forth an improvement in reducing the size of the apertures and, thereafter, returning the size of the apertures to the predetermined size. The improvement includes the following steps. A powder coating composition is applied to a shadow mask having apertures of the predetermined size in order to reduce the size of the apertures. The powder coating composition used is formed of a polymeric material having a molecular weight (31,) in the range of 10,000 to 35,000. The powder coating composition has an average particle size below 30 microns. The powder coating composition is applied to the shadow mask and the mask is heated with the material thereon to a temperature and for a time sufficient to sinter the material together but insufiicient to induce any substantial flow of the material. The polymeric material is effective to reduce the size of the apertures. After the color dot pattern is laid down by utilization of the shadow mask having the reduced sized apertures therein, the polymeric material is removed from the shadow mask and the shadow mask is used in the tubes final construction for directing the electron beam against the dots. The powders can be applied by any known technique used to coat metal articles by powder coatings. These techniques include electrostatic spray, fluidized bed, electrostatic fluidized bed, etc.

In greater detail, both thermoplastic and thermosetting polymeric materials which may be employed as the powder coating composition include, but are not limited to, polystyrene, polymethyl methacrylate, epoxy resins, cellulose acetate butyrate and copolymers of glycidyl acrylate or glycidyl methacryl'ate. To those polymeric ma terials may be added such materials as carbon black, an antistatic agent which aids in deposition of the powder on the shadow mask, and a thixotropic agent is desired.

Since the polymeric material applied to the shadow mask is heated to a temperature and for a time sufiicient only to sinter the particles of the material together, the material may be easily removed from the mask by common solvents. Thus, the predetermined size of the apertures may be reobtained with little difficulty after the dots have been placed on the tube. More particularly, since the polymeric material does not crosslink because of its limited heating, the material remains soluble in most common solvents. Thus, by utilization of a common solvent, the powder coating composition may be removed from the shadow mask and the mask. utilized in forming the final tube.

DESCRIPTION OF THE PREFERRED EMBODIMENT This invention relates to a method of reducing the size of apertures of a shadow mask which is employed in forming a color television tube. As is already well known in the art, the shadow mask has a plurality of small apertures therein, generally in the form of a circular opening, arranged in a predetermined pattern. These apertures have a diameter in the range of 14 to 16 mils. As is also known, each shadow mask is unique for a color tube in that it is utilized with the small apertures for laying down the color dot pattern on the face plate and with the larger apertures for use in directing an electron beam toward a particular dot on the face plate.

The improved method of this invention will be illustrated by a plurality of examples.

Example 1 One hundred parts by weight of polystyrene and 3 parts by weight carbon black are mixed together in a twin shell tumbling mixer for 10 minutes. The mixture is extruded through a compounding extruder at a temperature of 180 C. The extrudate is pulverized in a fluid energy mill so that it passes through a 270 mesh screen. This powder coating composition so produced has an average particle size of 15 microns.

The powder is sprayed onto a shadow mask which mask has a plurality of apertures aligned in a predetermined pattern. The diameter of these apertures is in the range of 14 to 16 mils. The powder is applied by an electrostatic spray gun operating at 60 kv. The coated shadow mask is baked at 140 C. for minutes. The size of the apertures is reduced to the range of 9 to 10 mils without any of the apertures being plugged. The apertures have circular shapes and smooth edges.

The shadow mask is utilized to lay down the groups of three different color dot phosphors on a face plate of a TV tube in a normal manner. After the color dot pattern is laid down, the powder coating composition is removed from the shadow mask by washing the same in toluene. The shadow mask is assembled in the tube by a plurality of manufacturing steps known in the art. The color tube is operated by receiving a color signal and the signal so derived is employed to excite the phosphor color dot pattern on the face plate. A color picture of acceptable quality is received on the color tube.

Example 2 A mixture containing 100 parts by weight of polystyrene (fi =15,000) and 2 parts carbon black are mixed together by shaking in a plastic jar. The materials are mill rolled at 180 C. for 20 minutes. The mixture is cooled and pulverized to pass through a 270 mesh screen. The mixture which passes through the mesh screen has an average particle size of about 10 microns.

The powder coating composition is sprayed onto a shadow mask by an electrostatic powder spray gun as described in Example 1 at an operating voltage of 40 kv. The coated shadow mask is baked at 120 C. for 5 minutes to sinter the polystyrene material together. This baking is insufficient to induce any substantial flow in the polymeric material. The size of the apertures in the shadow mask is reduced to a range of 9 to 10 mils with no plugging of holes of the shadow mask being noted.

A color TV tube is constructed and operated as described in Example 1 with the same results as described therein. Trichloroethylene is used as the solvent for removing the powder coating material from the shadow mask prior to its assembly to form the tube.

Example 3 A mixture containing 100 parts by weight of polystyrene (M =250,000) and 3 parts by weight of carbon black is prepared by the same procedure as described in Example 1. Because of the high molecular weight of the polystyrene, pulverizing is difficult and the average particle size of the final powder is 40 microns. The powder is applied to a shadow mask as described in Example 1 and the shadow mask is baked at 160 C. for 10 minutes. The size reduction of the apertures varies from 6 to 12 mils and some of the apertures are plugged. Also, many of the apertures have an irregular shape.

Because of the high molecular weight, the large particle size and the length of baking of the powder coating composition, the plugging and irregularity of the reduction of the apertures of the shadow mask is noted. Because of the plugging and irregularity of the apertures, the shadow mask cannot be utilized for laying down the color phosphor dot pattern on a face plate to obtain television tube of an acceptable quality.

Example 4 A mixture containing parts by weight of polymethylmethacrylate (M =25,000) and 3 parts by weight of carbon black is mixed together and prepared to form a powder in the same manner as described in Example 1. The average particle size produced is 20 microns.

The powder is sprayed on a shadow mask as described in Example 1. The shadow mask is baked at C. for 5 minutes. The apertures are reduced in size to a diameter of 9 to 10 mils with no plugging of the apertures noted.

A color picture tube is manufactured and operated as described in Example 1. An acceptable quality color picture is received on the tube. Prior to forming the complete color tube, the powder coating composition which has been sintered together in the heating step is removed from the shadow mask by the utilization of acetone.

Example 5 A mixture is formed of 100 parts by weight of polymethylmethacrylate (IWT,,:25,000), 1 part by weight carbon black, and 2 parts stearamidopropyldimethyl-B-hy droxyethylammonium nitrate which acts as an antistatic agent. The utilization of the two parts by Weight of the antistatic agent with the polymethylmethacrylate comes about because the carbon black percentage is reduced to 1 percent. The listed ingredients are mixed together in a twin shell tumbling mixer for 10 minutes and then extruded through a compounding extruder at a temperature of C. The extrudate is pulverized in a fluid energy mill to pass through a 270 mesh screen and the resulting powder has an average particle size of 15 microns.

The powder material is applied to a shadow mask in the same manner described in Example 1 and is baked at 150 C. for 5 minutes. The size of the apertures in the shadow mask is reduced to the range of 9 to 10 mils with no plugging of the apertures. A color TV tube is constructed and operated as described in Example 1. A good picture is received thereon.

Example 6 A mixture of 100 parts by weight polymethylmethacrylate (M =25,000) and 1 part by weight carbon black is processed in the same manner as described in Example 5 to achieve a powder coating composition having an average particle size of 15 microns. The material is applied to a shadow mask as described in Example 5 but the electrostatic deposition of this powder on the shadow mask is poor. The apertures are reduced in size to only 13 or 14 mils. The shadow mask is, therefore, not satisfactory for utilization in laying down the color phosphor dot pattern on a face plate of a tube.

Example 7 A mixture of 100 parts by weight epoxy resin, in this case Epon 1010, a trademark of the Shell Chemical Company, Inc., whose chemical formula is:

having a molecular weight equal to 10,000 is mixed with 3 parts by weight carbon black. The ingredients are mixed together in a twin shell tumbling mixer for minutes and extruded through a compounding extruder at a temperature of 175 C. The extrudate is pulverized in a fluid energy mill to pass through a 270 mesh screen. The powder so produced has an average particle size of 20 microns.

The powder is applied to a shadow mask by an electrostatic spray gun operating at 60 kv. and the coated shadow mask is baked at 150 C. for 5 minutes. The size of the apertures of the shadow mask is reduced to the range of 9 to 10 mils diameter and no plugging of the apertures exist.

A color picture tube is manufactured as described in Example 1. The tube is operated and the quality of the picture is acceptable. The epoxy resin is removed by a dichloromethane solvent in order to utilize the shadow mask in constructing the tube.

Example 8 The procedure of Example 7 is repeated with the sole exception that the material is baked at a temperature of 170 C. for 5 minutes. In this case, the higher baking temperature causes the powder coating composition to pass beyond a sintering stage and induces a substantial flow of the polymeric material out of the apertures. As a result of the flow of the material out of the apertures of the shadow mask, no size reduction is achieved and a laying down of a color dot pattern by use of the mask cannot be achieved.

Example 9 The same materials and processing conditions as described in Example 7 are repeated except that the powder coating composition applied to the shadow mask is baked at a temperature of 150 C. for a period of 20 minutes. The length of time of the baking operation causes the powder coating composition to pass beyond its sintering stage to a point whereat the powder coating composition has a substantial flow and it flows out of the masks apertures. Once again, the apertures are not reduced in size and the shadow mask cannot be used in laying down the color dot pattern.

Example 10 The procedure of Example 8 is repeated with the sole exception that 1 part by weight of fumed silica (sold under the trade name of Cab O-Sil M-S) is mixed with the epoxy resin and carbon black after the resin and carbon have been mixed together. This fumed silica acts as a thixotropic agent in modifying the viscosity of the powder when heated. When the powder is baked at a. temperature of 170 C. for 5 minutes, the powder does not reach a stage where there is substantial flow. Thus, a reduction of the apertures to a diameter of 9 to 10 mils is achieved without the plugging of any of the apertures. A color TV tube is constructed and operated as described in Example 1 and a color TV picture of satisfactory quality is received thereon.

Example 11 A mixture consisting of 100 parts by weight of cellulose acetate butyrate (CAB) is mixed with 3 parts by weight carbon black and a powder coating composition having an average particle size of 15 microns is prepared in the same manner as described in Example 1. The CAB used is a half-second butyrate having a melting point in the range of 155160 C.

The powder coating composition is applied as described in Example 1 and baked at a temperature of 150 C. for 5 minutes. By utilization of the material, a diameter reduction to the range of 9 to 10 mils for the apertures of the shadow mask is achieved and none of the apertures are plugged.

A color TV tube is constructed and operated as described in Example 1 by use of the shadow mask prepared. The powder coating composition which has been sintered together without the inducing of any substantial flow thereof is removed from the shadow mask by utilizing methylethyl ketone as the solvent. The color picture received on the tube is of satisfactory quality.

Example 12 One hundred parts by weight of a glycidyl methacrylate copolymer prepared by reacting 10 weight percent glycidyl methacrylate with 70 weight percent methyl methacrylate and 20 weight percent butyl methacrylate in a standard manner to produce a copolymer having a molecular weight of 10,000 is mixed with 3 percent by weight carbon black. The copolymer and carbon black are mixed and processed to form a powder coating composition having an average particle size of 25 microns in the same manner as described in Example 1.

The powder coating composition is applied to a shadow mask in the same manner as described in Example 1 and the shadow mask is baked at a temperature of C. for 5 minutes. The baking at this temperature for this time sinters the powder coating together in such a manner that the diameter of the apertures is reduced to the range of 9 to 10 mils and none of the apertures are plugged. A color TV tube is prepared and operated as described in Example 1 and a color picture of acceptable quality is received thereon.

What is claimed is: 1. In the method of making a metallic shadow mask for use in the manufacture of a color picture tube having a plurality of phosphor color dots placed on the face plate comprising the steps of making said metallic shadow mask with a plurality of apertures therein arranged in a predetermined manner each of said apertures having a predetermined size larger than the size of each of said phosphor color dots, reducing the predetermined size of each of said apertures to a smaller size by applying a coating, and

removing the applied coating from the coated shadow mask after said plurality of color dots have been placed on said face plate of said color picture tube to return the size of said apertures to said predetermined size prior to coating, wherein the improvement comprises:

electrostatically spraying said coating upon said shadow mask, said coating being a powder coating composition having an average particle size below 30 microns and comprising a synthetic polymeric material having a molecular weight (M in. the range of 10,000 to 35,000, and

heating said coated shadow mask to a temperature and for a time sufiicient to sinter said coated polymeric material together but insuflicient to induce any substantial flow of said coated polymeric material.

2. The improved method as defined in Claim 1 wherein: said powder coating composition contains a small weight percentage of an antistatic agent.

3. The improved method as defined in Claim 1 wherein: said step of removing comprises dissolving said coated polymeric material in a solvent therefor.

4. The improved method as defined in Claim 1 wherein: said powder coating composition comprises styrene and contains a small weight percentage of carbon black.

5. The improved method as defined in Claim 1 wherein: said powder coating composition comprises polymethylacrylate and an anti-static agent.

6. The method as defined in Claim 5 wherein: a portion of said anti-static agent is carbon black.

-7.*Theiiiethod as defined in Claim'l wherein:

said

polymeric material is an epoxy resin.

8. The method as defined in Claim 1 wherein:

said

polymeric material is cellulose acetate.

9. The method as defined in Claim 1 wherein:

said

pblyrneric material is a glycidyl acrylate or methacrylate copplymer.

References Cited UNITED Frantzen 1568 Arndt 96-36.1

Moegenb'ier 9636.1 Rucinshi 96-36.1 Black 11733.5 CM

Feldstein 9636.1

Lerner 96--36.1 Lance 11733.5 CM

10 MICHAEL SOFOCLEO-US, Primary Examiner US. Cl. X.R.

9636.1; 117-17.5, 21, 33.5 CM, 99; 31385 R, S, 92 B 

